This invention relates to capacitive position transducers and associated methods for position detection. The invention is particulary concerned with linear and rotary position transducers but is not limited to such transducers.
There is frequently a requirement in modern control systems for a positional transducer which provides a high resolution, absolute output, that is one in which the output indication is a unique expression of position. In applications where the accuracy requirements do not preclude their use, e.g. positional servo systems, potentiometers have traditionally been used, providing a readily available, cost effective solution where applicable.
However potentiometers do have several shortcomings. There is an inherent wear-out mechanism between the wiper and the resistive element which will ultimately result in failure of the device. They are prone to creating wiper noise, particularly under high rates of movement, and noise performance tends to deteriorate with life; this is a particular problem in high gain servo systems where the noise can interfere with correct system operation. Frequently the need is to monitor a linear motion and, whilst some linear travel potentiometers are available, these are almost invariably commercial parts unsuitable for anything other than consumer type applications. The application of a rotary potentiometer to such a requirement would therefore necessitate some kind of motional translationxe2x80x94a rack and pinion for example.
There is therefore a need for a potentiometer which obviates at least some of the above shortcomings. We have therefore designed a new form of transducer which does not require electrical physical contact between the mover and the stator and which provides an absolute output with high resolution, with an accuracy at least comparable with that of conventional servo-grade potentiometers.
Accordingly, in one aspect, this invention provides a capacitive position transducer comprising:
a relatively fixed substrate including two spaced electrically conductive regions;
a mover element comprising a common electrical pick-off means capacitively coupled to both of said electrically conductive regions and mounted for movement relative to said substrate in a sensing direction, the transverse extent of at least one of said electrically conductive regions varying with the position of said mover element in the sensing direction,
drive means for supplying respective different periodic time varying voltages to said electrically conductive regions, and
processing means for processing the electrical signal received by said pick-off means to determine the position of said mover element,
wherein said drive means applies respective different sinusoidal voltages to said electrically conductive regions, said sinusoidal voltages having a relative phase difference.
Preferably, the processing means determines at least one of the phase and magnitude of the electrical signal received by the pick-off means, to determine the relative position of the mover element.
In another aspect, this invention provides a capacitive position transducer comprising:
a relatively fixed substrate including two spaced electrically conductive regions;
a mover element comprising a common electrical pick-off means capacitively coupled to both of said electrically conductive regions, and mounted for movement in a sensing direction, the electrically conductive regions being arranged such that at least one of the respective capacitances between said pick-off means and the electrically conductive regions varies with the position of said mover element in said sensing direction, and
drive means for applying respective voltages to each of said electrically conductive regions, and
position determining means for monitoring the position of said mover element relative to said substrate,
wherein said drive means supplies respective different sinusoidal voltages to said electrically conductive regions, said sinusoidal voltages having a relative phase difference.
In yet another aspect, this invention provides a method of position detection which comprises providing a relatively fixed substrate including two spaced electrically conducting regions and a pick-off means capacitively coupled to both of said regions, said pick-off being connected to a mover element for movement in a sensing direction, at least one of the respective capacitances between the pick-off and the electrically conductive regions varying with position in said sensing direction, the method further comprising monitoring at least one of said capacitances thereby to determine the position of said mover element, and
applying respective different sinusoidal voltages to said electrically conductive regions, said sinusoidal voltages having a relative phase difference.
Preferably, both of said electrically conducting regions vary in transverse extent in said sensing direction whereby the capacitance between each electrically conductive region and the pick-off means varies in said sensing direction.
Preferably, said electrically conductive regions vary linearly and inversely in transverse extent in said sensing direction. Thus, in one arrangement the electrically conductive regions may be arranged as two triangles in inverted relationship together defining a generally rectangular plan shape.
Alternatively, the electrically conductive regions may each vary in said sensing direction with a profile selected in accordance with the drive voltages to said electrically conducting regions, at least partially to compensate for non-linearities in the output signal.
The pick-off means may be connected directly to said processing means e.g. via an electrically conducting wire or the like. Alternatively, to avoid the need to provide a moveable electrically conducting element such as a wire or track, the pick-off means may be capacitively coupled to a track means which extends alongside the path of movement of said mover element, with suitable screening, so that the output signal may be taken from the track means.
The substrate may take any of a number of different forms. For example, the substrate may be generally flat or planar. Here the mover element may be mounted for linear movement in a straight line relative to said substrate, analogous to a conventional linear potentiometer. Alternatively, said mover element may be mounted for rotary movement about a rotary axis and the substrate may be of generally disc or annular form. In this instance the transducer is akin to a rotary potentiometer.
Alternatively, said substrate may comprise a cylindrical or part-cylindrical surface and said mover element may be disposed adjacent said cylindrical a part-cylindrical surface and mounted for movement about an axis generally coaxial with the principal axis of said cylindrical or part cylindrical surface.
The drive means may apply various forms of drive voltages to the electrically conductive regions such that the combined signal at the electrical pick-off means may be processed to determine the position of the mover element. Thus the drive means may apply respective different sinusoidal voltages. The sinusoidal voltages are preferably of substantially equal amplitude. Conveniently, said sinusoidal voltages have a relative phase difference of 90xc2x0, effectively to provide respective sin and cosine waveforms. However other phase differences may also be used.
The processing means preferably determines at least one of the relative phase and magnitude of the electrical signal received by said pick-off means, to determine the position of said mover element relative to a fixed datum position.
Where the electrically conductive regions vary linearly and inversely in axial extent in said sensing direction, the drive means may apply respective sinusoidal waveforms of the form Vxc2x7Sin(xcfx89t) and Vxc2x7Cos(xcfx89t) and the relative position of the mover element may be determined from the signal Vo at said pick-off, wherein
Vo=Kxc2x7Vxc2x7Sin(xcfx89txcex8);
xcex8=arctan(xcex1/1xe2x88x92xcex1);
K={square root over ( )}(xcex12+(1xe2x88x92xcex1)2)
and xcex1 is the distance of the mover element from the highest capacitance part of the electrically conducting region driven by said Vxc2x7Sin(xcfx89t) waveform expressed as a proportion of the maximum range of sensing movement of said mover element.
It will be noted that xcex1 and xcex8 vary non-linearly; this may be overcome by modifying the profiles of the electrically conductive regions to reduce or remove the non-linearity so that the phase difference (xcex8) varies linearly with the position (xcex1) of the mover element.
Alternatively, the non linearity may be overcome by signal processing. Thus said processing means may be operable to multiply said output voltage Vo at said pick-off means with a D.C.-nulling periodic time varying voltage at the same frequency as said output voltage, and adjusting the relative phase of said D.C.-nulling periodic time varying voltage to null the D.C. term of said product, and thereby determine the position of said mover element relative to a fixed datum.
Preferably said D.C.-nulling periodic voltage is generated by multiplying the drive voltages applied to said electrically conducting regions. Thus said processing means may include generating means for producing said D.C.-nulling periodic voltage, said generating means including means for multiplying the time varying voltages (Vxc2x7Sin(xcfx89t); Vxc2x7Cos(xcfx89t)) supplied to said electrically conductive regions with respective inversely related voltages, (Vrefxe2x88x92Vcon) and (Vcon), means for summing the product of said multiplying means and phase shifting the sum to obtain said D.C.-nulling periodic voltage.
Preferably said processing means includes integrator means for receiving the product of said pick-off output voltage Vo with the D.C.-nulling voltage Vxe2x80x2o, and adjusting said inversely related voltages, (Vrefxe2x88x92Vcon) and (Vcon) to null said D.C. term.
In yet another aspect, there is provided a capacitive position transducer comprising:
a relatively fixed substrate including two spaced electrically conductive regions varying lineally and inversely in transverse extent;
a mover element comprising a common electrical pick-off means capacitively coupled to both of said electrically conductive regions and mounted for movement relative to said substrate in a sensing direction, the transverse extent of at least one of said electrically conductive regions varying with the position of said mover element in the sensing direction,
drive means for supplying respective different periodic time varying voltages to said electrical conductive regions, and
processing means operable to multiply an output voltage at said pick-off means with a DC nulling periodic voltage at the same frequency as said output voltage and to adjust the relative phases of said DC nulling periodic time varying voltage to null the DC term of said product, and thereby to determine the relative position of said mover element.
Whilst the invention has been described above, it extends to any inventive combination of the features set out above or in the following description.